It is prior art technology to separate (or classify) particles in cyclones that utilize centrifugal force (centripetal force) to separate small particles from larger particles as effectively as possible. Apparatuses that are suited for the purpose are so-called “forced vortex classifiers” which comprise a rotor with blades between which the particles must pass to escape with the fine fraction. All the larger particles are here effectively separated out.
It is, however, a problem to quantitatively separate all the fine particles from the coarse fraction. In the discussion below we distinguish between two types of forced vortex classifiers, namely the “free rotor” type and the “stationary vane” type.
The free rotor type has a rotor that far from fills the diameter of the house in which it is mounted which renders it possible for a flow of particles to be charged pneumatically and well dispersed in a flow of air to the outside of the rotor periphery. This is desirable e.g. because it is effective with respect to the volume of air/particles that are able to pass through in a certain period of time and because it leads to comparatively low rotor wear etc. from the larger particles that never come in contact with the rotor. It has unfortunately the disadvantage that a certain amount of the finer particles may stick to the surface of larger particles and thus leave with the coarse fraction.
In a stationary vane classifier there is only a small gap between the rotor and a rim of stationary blades or vanes that are arranged like a circumferential rolling door around the rotor and through which air is fed to the classifier. The particles (material) that are to be classified are charged to the classifier without air or with only a little air to disperse the particles. The particles belonging to the coarse fraction are generally moving in a direction downwards within the classifier in the general direction of the outlet for this fraction, and are along their way several times “knocked” back and forth between the stationary vanes and the outer part of the rotor blades, while t the same time being exposed to a severe turbulence. Due to these impacts the fine particles that stick to the larger particles are released and thereby allowed to follow the fine fraction up between the rotor blades. The advantage with the stationary vane classifier is that the fine fraction is more effectively separated from the coarse fraction.
While the free rotor type is quite effective with respect to charge of a mixture of air and particles in a dispersed state but less effective with respect to separate the fine particles from the coarse fraction, the stationary vane classifier is superior with respect to separate fine particles from the coarse fraction but has a comparatively lower capacity with respect to volume/time unit that may be charges thereto, due to its less effective dispersion ability.